The main reason is because it doesn't defend Tea's life points.
Cards like Lady Assailant of Flames have a flip effect which deals damage.
The difference is the Opponent has to use 1 monster to kill the Face Down Lady Assailant.
Skystarray leaves the field after it fights.
It leave Tea open for direct attack.
Tea burn decks are usually Stalling Burn Decks.

Could this card be used as a substitute to vassal if you dont have "secret pass to the treasure" or "shooting star bow - ceal".
And how good is combining with "gift of the martyr" since i dont have "union attack" nor "riryoku".
I actually use it for farming to acheive 5k+, combining two "gift of martyrs" (tribute BEWD and high level ritual).
Please give opinion on this.

no it isn't. Vassal is used because of his effect that changes battle damage to effect damage, which is why union attack works with vassal to actually deal damage. Using union attack on this thing won't make it deal damage.

But basically, use cerebrus to accumulate ATK, and last turn summon skystarray and use gift of martyr, and only thing you get less is 1000 points less because there is no effect damage, right? and you do not need riryoku, union atk or similar here. Right?
I mean, this is the only solution left cause secret pass is not available anymore?